[SI-LIST] R: Re: R: LVDS Transmission Problem
- From: "Guasti Giovanni" <Giovanni.Guasti@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 27 Sep 2005 18:13:23 +0200
Hi Hassan,
I did a very simple calculation, maybe I'm wrong: an LVDS differential =
swing at the driver is about 800mV and 150m of cat5 cable is about =
20-30dB of losses @30-100MHz. So at the receiver we'll have ABOUT =
80-24mV of swing, that is under or near the threshold (100mV). Are you =
confident about using an LVDS with 150m of cable?
In the National (DS90LV032ATM & DS90LV031ATM) appl. note 971 there is =
also a picture of the signal after 10 m of cable @ 155Mb/s. Signal is =
quite good, but rise and fall time increase a lot.
Anyway the worst contribute to the signal in this particular case is =
given by connectors, flexes and the backplane.
That's why I think that the LVDS is not the FIRST choice. I think that =
the noise margin could be reduced too much.
Best regards,
Giovanni
-----Messaggio originale-----
Da: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]Per conto di Hassan O. Ali
Inviato: marted=EC 27 settembre 2005 16.54
A: si-list@xxxxxxxxxxxxx
Oggetto: [SI-LIST] Re: R: LVDS Transmission Problem
Cable loss is certainly not the problem here. Plain old LVDS signaling =
can be used with=20
24 gauge twisted-pair cables even longer than 150 meters! One can refer =
to the cable=20
guidelines in the LVDS standard page 25 (http://ftp.tiaonline.org/tr-
30/tr302/Public/2000%20Contributions/20005017.pdf)
I think the problem here could be with the data pattern, data rate, or =
the type of=20
connection. Relevant questions to consider:
- Is it a dc or ac-coupled link?=20
- Does the failing data pattern have long runs of zero's and one's that =
considerably=20
alter the dc balance of the signal? More so if ac-coupled.
- Is 34MHz below the low frequency limit of the LVDS receiver/driver?
Best regards.
Hassan.
On Sep 27, "Guasti Giovanni" <Giovanni.Guasti@xxxxxxxxxx> wrote:
>=20
> I think LVDS is not the first choice for a so long path.
> How is the measured signal? Is a problem of losses, reflections or a =
=3D
> signal polarization due to distributed capacitance?
> maybe the LVDS suffers the high capacity of the system (maybe is just =
=3D
> the shield).
> After the signal measurement, I would try with these:
> 1)a scrambled (or a more balanced number of '1' and '0')signal=3D20
> 2)a unshielded cable / unshilded and shorter cable.
> 3)replace LVDS with a LVPECL or MLVDS/BLVDS
> I hope this could help you,
> Giovanni
>=20
>=20
> Hi=3D20
>=20
> =3D20
>=20
> We have a system which is build up two subsytem. Data signals between =
=3D
> the
> subsystems are in LVDS format. Between these two subsystems there is =
10m
> cable. LVDS data signals are carried using shielded twisted pair =
cables.
> Inside these two subsystem LVDS signals also passed through backplanes =
=3D
> and
> flexes . We use national LVDS transmitter and receiver. (DS90LV032ATM =
&
> DS90LV031ATM) . The data rate of the LVDS signals are 34MHz. The =
problem =3D
> is
> for some data patterns the transmitted and received data is not same? =
=3D
> As
> the cable length is shortened to 3m and frequency of LVDS signals is
> reduced the problem somewhat lowered but we have still the problem. =
What
> could be the reasons for this problem?
>=20
> =3D20
>=20
> <mailto:r@xxxxxxxxxxxxxxxxxxx>=3D20
>=20
> M.Baris DUZGUN
>=20
> Hardware Senior Expert Engineer
>=20
> ASELSAN INC. MGEO Division
>=20
> P.K. 30 06011
>=20
> Etlik ANKARA
>=20
> TURKEY
>=20
> Tel : +90 312 847 53 00 / 4534
>=20
> Fax : +90 312 847 53 20
>=20
> e-mail : bduzgun@xxxxxxxxxxxxxxxxxxx =3D
> <mailto:bduzgunr@xxxxxxxxxxxxxxxxxxx>=3D20
>=20
> =3D20
>=20
>=20
We have a system which is build up two subsytem. Data =3D
> =3D3D
> signals
> between the subsystems are in LVDS format. Between these two =
subsystems =3D
> =3D3D
> there
> is 10m cable. LVDS data signals are carried using shielded twisted =
pair =3D
> =3D3D
> cables.
> Inside these two subsystem LVDS signals also passed through backplanes =
=3D
> =3D3D
> and flexes
> . We use national LVDS transmitter and receiver. (DS90LV032ATM &
> DS90LV031ATM) . The data rate of the LVDS signals are 34MHz. The =
problem =3D
> =3D3D
> is for
> some data patterns the transmitted and received data is not =
same? =3D
> =3D3D
> As the cable
> length is shortened to 3m and frequency of LVDS signals is reduced the =
=3D
> =3D3D
> problem
> somewhat lowered but we have still the problem. What could be the =
=3D3D
> reasons for
> this problem?
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